For sensory systems, feedforward projections from thalamic relay cells provide the cortex with information about the external environment. The cortex, in turn, sends extensive feedback to the thalamus. The cortex thus functions both to process information supplied by the thalamus as well as to influence dynamically the transmission of thalamic input. The primary goal of the experiments presented in this proposal is to determine how the primary visual cortex (V1) and the lateral geniculate nucleus (LGN) of the thalamus interact to dynamically influence visual processing. The study involves three sets of experiments. The first major series of experiments (Specific Aim 1) will test the hypothesis that corticogeniculate feedback serves to increase the strength and temporal precision of LGN responses to visual stimuli. The remaining experiments go on to test the exciting possibility that corticogeniculate feedback dynamically adjusts feedforward activity and functional connectivity to meet the behavioral and perceptual demands that occur during visual processing. The second major series of experiments (Specific Aim 2) will test the hypothesis that directed attention influences the dynamic properties of feedforward and feedback pathways interconnecting the LGN and V1. The third major series of experiments (Specific Aim 3) will test the hypothesis that perceptual modulation during binocular rivalry influences neuronal activity in the LGN and the dynamic properties of feedforward and feedback connectivity. Given the central importance of corticothalamic pathways for governing the excitability of thalamocortical networks, it is important that we understand the functional properties of the corticothalamic pathway, as disorders of the pathway likely underlie several illnesses affecting vision and visual processing. PUBLIC HEALTH RELEVANCE: A reciprocal arrangement of neuronal connections governs the excitability of neurons in the thalamus and cerebral cortex. Given the severe financial and quality-of-life consequences that follow from disruption in the ability of these brain regions to communicate with each other, such as occurs with many forms of epilepsy and several illnesses affecting vision and visual processing, it is important that we gain a better understanding of thei functional relationship. The goal of this proposal is to determine how the cortex and thalamus interact to dynamically influence visual processing.